The Solar System!
The solar system consists of the Sun; the eight official planets, at least three "dwarf planets", more than 130 satellites
of the planets, a large number of small bodies (the comets and asteroids), and the interplanetary medium.
(There are probably also many more planetary satellites that have not yet been discovered.)
The inner solar system contains the Sun, Mercury, Venus, Earth and Mars.
The main asteroid belt (not shown) lies between the orbits of Mars and Jupiter. The planets of the outer solar system are
Jupiter, Saturn, Uranus, and Neptune (Pluto is now classified as a dwarf planet).
The first thing to notice is that the solar system is mostly empty space. The planets are very small compared to the space
between them. Even the dots on the diagrams above are too big to be in proper scale with respect to the sizes of the orbits.
The orbits of the planets are ellipses with the Sun at one focus, though all except Mercury are very nearly circular. The
orbits of the planets are all more or less in the same plane (called the ecliptic and defined by the plane of the Earth's
orbit). The ecliptic is inclined only 7 degrees from the plane of the Sun's equator. Of the eight planets (plus Pluto), they all orbit in the same direction (counter-clockwise looking down from above
the Sun's north pole); all but Venus, Uranus and Pluto also rotate in that same sense.
One way to help visualize the relative sizes in the solar system is to imagine a model in which everything is reduced in
size by a factor of a billion. Then the model Earth would be about 1.3 cm in diameter (the size of a grape). The Moon would
be about 30 cm (about a foot) from the Earth. The Sun would be 1.5 meters in diameter (about the height of a man) and 150
meters (about a city block) from the Earth. Jupiter would be 15 cm in diameter (the size of a large grapefruit) and 5
blocks away from the Sun. Saturn (the size of an orange) would be 10 blocks away; Uranus and Neptune (lemons) 20 and 30
blocks away. A human on this scale would be the size of an atom but the nearest star would be over 40000 km away.
Their are numerous smaller bodies that inhabit the solar system: the satellites of the
planets; the large number of asteroids (small rocky bodies) orbiting the Sun, mostly between Mars and Jupiter but also
elsewhere; the comets (small icy bodies) which come and go from the inner parts of the solar system in highly elongated
orbits and at random orientations to the ecliptic; and the many small icy bodies beyond Neptune in the Kuiper Belt.
With a few exceptions, the planetary satellites orbit in the same sense as the planets and approximately in the plane of
the ecliptic but this is not generally true for comets and asteroids.
